1. Field of the Invention
This invention relates generally to a Code Division Multiple Access (CDMA) mobile communication system, and more particularly, to a method for increasing the probability of the success of handoff service in the CDMA mobile communication system in which, when a mobile station enter a handoff service region, the mobile station continue to transmit with a regular interval a pilot strength measurement message (PSMM) to a base station to request a handoff service until the mobile station receives the handoff service if it does not so during its stay within the handoff service region and the base station holds back a new call service and assigns a released channel to the mobile station requesting repeatedly the handoff service if a released call is present.
2. Description of the Related Art service provided in a DS-CDMA (Direct Sequence-CDMA) implemented based on a IS-95 (International Standard-95) includes a soft handoff process and a hard handoff process.
The soft handoff process is a process that a mobile station is assigned simultaneously with traffic channels from two base stations while the mobile station is located between an in-service base station and its neighboring base stations. On the other hands, the hard handoff process is to be used when the frequency used in the in-service base station is different from that used in the neighboring base stations.
Such conventional handoff processes have been developed for the improvement of the voice quality over an analog process by introducing a soft handoff technique among sectors, among base stations, and among controllers of the base stations.
FIG. 1 is a diagram for explaining a relation of a pilot strength level to a distance between the base stations in the CDMA cellular system, wherein, as the mobile station moves from a service (or original) base station to a neighboring base station, the strength of the pilot signal from the service base station is reduced but the strength of the pilot signal from the neighboring base station is increased.
The pilot strength measurement message (PSMM) is a message for the measurement of the strength of the pilot signal received by a base station/base station controller from the mobile station and a handoff direction message (HDM) is a message used when the base station/base station controller requests the handoff to the mobile station.
A handoff completion message is a message which the mobile station transmits to the base station/base station controller and used for an acknowledge of the handoff direction message (HDM).
Parameters for the measurement of the pilot strength are as follows:
T_ADD is a value of the pilot strength required for the pilot signal to be a candidate set.
The mobile station transmits the PSMM message to the base station when the pilot strength of the candidate set is more by T_COMP than that of the active set.
T_DROP is a value of a minimum signal level for maintaining the base station in the active set.
T_TDROP is a value of a channel drop timer required to delete pilots belonging to the active set from the active set.
ADD means that pilots of the neighboring sectors or cells are added within the active set when the mobile station moves toward the neighboring sectors or cells in the CDMA handoff process. DROP means that pilots within the active set are deleted from the active set in the CDMA handoff process.
If the strength of the pilot signal in the second base station is greater than T_ADD when the mobile station moves from the service base station to a neighboring base station, the mobile station transmits the PSMM message to the neighboring base station through the base station controller that controls both the service base station and the neighboring base station. And then, the neighboring base station reserves an idle traffic channel and reports it to the base station controller.
The base station controller assigns the traffic channel of the neighboring base station to the mobile station through the handoff direction message (HDM) and instructs that the pilot of the neighboring base station is included in the active set. At that time, the mobile station forms communication pass simultaneously with two base stations (the service base station and the neighboring base station).
When the strength of the pilot signal of the first base station is reduced below T_DROP as the mobile station becomes closer to the neighboring base station, the mobile station activates a timer to measure the pilot strength of the service base station after a lapse of T_TDROPS sec. If the pilot strength of the base station remains below T_DROP, the PSMM message is transmitted to the control station in order to release the traffic channel with the base station 1.
The control station which has received the PSMM message releases the traffic channel in talk with the first base station by transmitting the handoff direction message to the mobile station to end the handoff.
A region in which the channel is released between T_TDROP point and T_TDROP point as shown in FIG. 1 is referred to a soft handoff region.
If the mobile station does not receive any handoff service at T_ADD point due to high traffic load in the second base station, the handoff service is held back until T_COMP point.
Also, as the mobile station which does not receive the handoff service at T_ADD becomes more distant from the first base station during the approach to T_COMP point, a call outage may occur when a radio wave environment becomes deteriorated. Accordingly, there are problems in that the failure of the handoff service is caused if the mobile station does not receive the handoff service at T_COMP point and the call is discontinued when a signal from the first base station becomes weak.
In consideration of the above problems, it is an object of the present invention to provide a handoff method for improving the talk quality in which a probability of the handoff success is raised and a probability of a call outage to disconnect a busy call is reduced by transmitting a pilot strength measurement message (PSMM) successively with a regular interval from a mobile station to a service base station to request a handoff service until the mobile station receives the handoff service if it does not so during its stay within a handoff service region when the mobile station enter the handoff service region, holding back a new call service from the base station and assigning a released channel to the mobile station requesting repeatedly the handoff service if a released call is present.
In order to accomplish the object, according to the present invention, a handoff method applicable to a mobile communication system comprises a first step of requesting a handoff service repeatedly with a regular interval based on a timer from a mobile station to a service base station until the mobile station receives the handoff service if a pilot strength of a neighboring base station is greater than a base station pilot strength value (T-ADD_when the mobile station enters a handoff service region; and a second step of holding back a new call service in the neighboring base station if the mobile station requesting the handoff service does not receive the handoff service and assigning a released channel to the mobile station requesting repeatedly the handoff service if a released call is present.